Optical regeneration circuit

ABSTRACT

An optical regenerator includes a clock recovery circuit which recovers an optical clock signal from a received deteriorated optical data signal. The recovered optical clock signal is used to generate a new optical clock signal. The new optical clock signal and the received deteriorated optical data signal are applied as an input signal and control signal, respectively, to a Sagnac switch which encodes the new optical clock signal with the encoded information of the deteriorated optical data signal thereby regenerating a clean optical data signal.

BACKGROUND OF THE INVENTION

This invention relates to optical communication circuits and, moreparticularly, to an all-optical repeater for use in an opticalcommunication system.

The most important feature of optical digital signal transmission is theability to reconstruct the transmitted pulse train after it has traveledthrough a dispersive and noisy medium (free-space, optical fiber, etc.).This process of reconstructing the pulse train is performed at intervalsalong the transmission path by regenerative repeaters.

In prior art optical communication system repeaters, received opticalsignals are converted to electrical signals and processed usingwell-known electronic circuits and then convened to an optical signalfor transmission. As bit rates increase, the expense and complexity ofperforming regeneration electronically rise dramatically. The initialdesign is much more difficult at high speeds, and the extremereliability required of telecommunications repeaters compounds the cost.Thus, there is a continuing need to improve the performance of opticalregenerators.

SUMMARY OF THE INVENTION

We have recognized that the transfer characteristics of a Sagnac loopprovide a limiter function which could be utilized in an opticalregenerator. Moreover, we have recognized that a Sagnac loop could,desirably, be used to reduce timing jitter, amplitude jitter, andwavelength chirp in an optical regenerator. In accordance with ourinvention, an optical regenerator includes a clock recovery circuitwhich recovers an optical clock signal from a received deterioratedoptical data signal. The recovered optical clock signal is used togenerate a new optical clock signal. The new optical clock signal andthe deteriorated data signal are applied as an input signal and controlsignal, respectively, to a Sagnac switch which regenerates an opticaldata signal from the deteriorated data signal. More specifically, theSagnac uses the deteriorated data signal to encode the new clock signaland thereby regenerate a clean optical data signal.

The use of an "all-optical" regenerator makes possible the regenerationof telecommunications signals with much less complexity, and usesoptical components which are much easier to qualify for high-reliabilityoperation. Even if electronics is used for the narrowband portion of thecircuit (clock recovery), the ultra-wide bandwidth of optical processingmakes it deskable for the broadband portion (regeneration).

According to one embodiment of the invention, the wavelength of the newclock signal is different from the wavelength of the receiveddeteriorated data signal. Hence in such an embodiment, the wavelength ofthe regenerated optical data signal is different from the wavelength ofthe deteriorated data signal. An optical communication system isimplemented using an optical communication medium and using our opticalregenerator as part of an optical repeater or receiver.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing

FIG. 1 shows a block diagram of an illustrative optical communicationsystem including a repeater implemented in accordance with the presentinvention and

FIG. 2 shows illustrative signal waveforms useful in understanding thepresent invention.

DETAILED DESCRIPTION

In the following description, each item or block of each figure has areference designation associated therewith, the first number of whichrefers to the figure in which that item is first located (e.g., 130 islocated in FIG. 1 and wave form 202 is located in FIG. 2).

Shown in FIG. 1 is a block diagram of an illustrative opticalcommunication system including an optical data transmitter 110; one ormore repeater links including optical fiber link 111 and optical datarepeater 120; optical fiber link 145; and optical data receiver 160. Theoptical repeater 120 as well as optical data receiver 160 may includeour optical regenerator 150 for regenerating optical data signals. In awell-known manner, optical data transmitter 110 generates optical dataoutput signal 202 shown in FIG. 2 from the electrical input data signal201 on input data lead 109. Optical fiber link 111 is a dispersive andnoisy medium which distorts the transmitted optical data signal 202producing the distorted optical data signal 203 which is inputted torepeater 120. Repeater 120 uses regenerator 150 to regenerate theoptical data signal, as shown by 204, for its subsequent transmissionover optical fiber link 145 where data receiver 160 detects the receivedoptical data signal and converts it to electrical signal 205 on dataoutput lead 163.

The use of an "all-optical" regenerator 150 makes possible theregeneration of telecommunications signals with much less complexity anduses optical components (laser 125, isolator 128, Sagnac switch 130 andwavelength filter 142), which are much easier to qualify forhigh-reliability operation. Even if electronics is used for thenarrowband portion of the circuit (clock recovery circuit 121), theultra-wide bandwidth of optical processing makes it desirable for thebroadband portion (Sagnac switch 130 performs the regeneration).

Optical data regenerator 150, illustratively, may include a clockrecovery circuit 121, laser 125 and Sagnac switch 130. The well-knownclock recovery circuit 121, illustratively includes an optical signaldetector 122 and a band-pass filter 123 to recover a clock signal fromthe received optical data signal 111. Optical signal detector 122converts the received optical data signal 111 to an electrical datasignal which is then filtered by band-pass filter 123 to reconstruct asinusoidal clock signal 124 at the received data rate.

Band-pass filter 123 includes an optional electronic amplifier (notshown) to adjust the electrical signal level for the modelocked laser125. By using a hi-Q filter, a sinusoidal signal is produced which bearsa fixed phase relationship to the incoming data 111, and which tracksany wander in the frequency or phase of the transmitted data stream. Thesinusoidal clock signal 124 from the band-pass filter 123 is used tomodelock an actively modelocked laser 125 which generates a new opticalclock signal 126. The optical clock signal 126 passes through isolator128.

The new or recovered optical clock signal 129 and received optical datasignal 127 provide inputs to Sagnac switch 130. Such a Sagnac switch isdescribed in more detail in the commonly assigned U.S. Pat. No.5,144,375, issued Sep. 1, 1992 to Gabriel, Houh and Whitaker entitled"Sagnac Optical Logic Gate," which is incorporated by reference herein.

We have recognized that the Sagnac switch 130 provides severalcharacteristics which are desirable for use in optical repeater 120. Forexample, as will be described in more detail in later paragraphs, Sagnacswitch 130 is insensitive to timing jitter and amplitude jitter.Additionally, the AND logic function performed by Sagnac switch 130enables the optical repeater to encode the clock signal 129 with theinformation content of the received data signal 127. Furthermore, theSagnac switch 130 eliminates wavelength chirp (the frequency dispersionof an optical pulse). The resulting repeater 120 thus possessesdesirable timing jitter, amplitude jitter, wavelength chirp and pulseshape restoration characteristics. Optical repeater 120 effectivelyregenerates a data output signal over fiber link 145 having the originalcharacteristics of the data signal transmitted from data transmitter110.

Sagnac switch 130, illustratively, includes an optical transmissionmedium, i.e., optical fiber 131, that is connected at both ends to aPolarization Maintaining Coupler (PMC) 132. An input optical clocksignal 129 is applied to an input SI which is port 1 of coupler 132.Ports 2 and 4 of coupler 132 are connected to the two ends of fiber 131,and port 3 of coupler 132 forms an output SO of the Sagnat loop. Fiber131 thus forms a loop, (also referred to herein as fiber loop 131) whichin the context of this disclosure, refers to the path over which asignal travels and, more particularly, to arrangements where the pathforms a closed, or nearly closed, figure.

The Sagnat switch 130 operates as follows. Clock signal 129 is appliedto port 1 and is split into two parts that exit coupler 132 at ports 2and 4: a "ref" signal that travels clockwise, and a "ref" signal(reference signal) that travels counter-clockwise. The "ref" and "ref"signals travel through the loop in opposite directions, re-enter coupler132 and recombine therein. Under normal circumstances, the "mark" and"ref" signals experience the same conditions as they travel through theloop. Even though the propagation speed is a function of many parametersthat may be uncontrollable and may or may not change with time, thetravel time of the "ref" and "mark" signals is short enough that,basically, all of the parameters remain static. Consequently, no changesoccur within the loop to differentiate between the effects of the fiberon the signals traveling in the two directions. The result is acombining of signals in coupler 132 that is constructive with respect toport 1 and destructive with respect to port 3. In consequence, lightthat enters port 1 of coupler 132 is completely reflected back to port1, and no output is delivered to port 3.

In addition to the above-described structure, Sagnat switch 130 includesa wavelength combining coupler 135 that injects a control signal 127 atport CTLI into a segment 136 of the fiber loop 131. Because coupler 135is within loop 131, the control signal travels along loop 131 only inone direction; and more specifically, coupler 135 is arranged to injectthe control signal that travels along loop 131 in the direction of the"ref" signal. A wavelength combining coupler 137 may also be includedwithin the loop of fiber 131 to extract the pulse out of the loop onceit has served its control function.

Segment 136 of fiber 131 is a variable refractive-index material that ischaracterized by the property that the propagation speed of a beampassing through the material is a function of the intensity of the beamthat passes through the material. Furtherefore, not only does thepropagation speed change for the beam (e.g., control signal) thateffects the change in propagation speed, but it also changes thepropagation speed of other beams (e.g., "mark" signal) that pass throughthe material at the same time. The nonlinear interaction between thecontrol signal and the "mark" signal is by means of cross-phasemodulation due to the optical Kerr effect. Of course, the entire lengthof fiber 131 may be made of such a variable refractive-index material,but for the sake of generality, fiber 131 is drawn as having only alimited segment 136 being made up of this material. Also for the sake ofgenerality, it should be pointed out that the loop of fiber 131 in FIG.1 does not necessarily have to be fiber. It can be a waveguide, or othermeans for directing the flow of light.

In sum, the Sagnat switch 130 includes a fiber loop 131 having acontrollable propagation speed material in segment 136, "mark" and "ref"signals traveling through the loop in opposite directions and combinedin coupler 132 and a control signal (data signal 127) that is injectedat coupler 135 which travels in the same direction as the "mark" signalover segment 136 and extracted by coupler 137. When the "mark" andcontrol signals are properly timed and conditioned, the result is asingle-pole double-throw switch type apparatus. When the control signalis not present, the "mark" and "ref" signals are combined in coupler 132as described above. The signal entering the switch at port 1 reflectsout of the loop of fiber 131 and exits port 1 of coupler 132. However,when the control signal is present and is made to travel through segment136 with the "mark" signal, the change in propagation speed of the"mark" signal that is caused by the control signal alters the phase ofthe "mark" signal arriving at coupler 132. When the energy in thecontrol signal and the interaction internal within segment 136 (betweenthe "mark" and control signals) are properly controlled, the resultingphase relationship between the "mark" and "ref" signals is approximatelyπ radians, meaning that the "mark" signal is about 180° out-of-phase tothe "ref" signal. This causes the combining of the "mark" signal and the"ref" signal in coupler 132 to be completely destructive with respect toport 1 and completely constructive with respect to port 3. As a result,all of the energy exits at port 3 (non-reflected signal output port)rather than at port 1 (reflected signal output port). It may be noted inpassing that the "ref" signal also passes through segment 136 and thatits speed is also somewhat affected by the control signal. But, sincethe control signal and the "ref" signal travel in opposite directions,their interaction time is much shorter than the interaction time of the"mark" and the control signals.

To ensure the proper operation of the Sagnac switch (that is to minimizethe distortion of the pulse which outputs at port 3) requires that thecontrol signal completely traverse the "mark" signal during its transitthrough segment 136 of the fiber loop 131. This is accomplished byrequiting that the material of segment 136 have a dual speedcharacteristic, one that propagates the control signal at a differentrate than the "mark" signal. The difference in the propagation speed maybe tied to any controllable parameter of the control signal, such aswavelength, intensity or polarization. In the illustrative embodimentdifferent wavelengths are utilized. That is, the wavelength of laser 125clock signal is different than the wavelength of the input data signal127.

As long as the input data signal 127, which acts as the control signal,completely traverses the "mark" signal (generated from laser 125 clocksignal) within segment 136 the Sagnac switch operation is completelyinsensitive to the shape of the control signal or its precise timing.Rather, it is only sensitive to the overall energy of the control signal(integral of the control pulse).

In the present embodiment, since the control signal is actually theinput data signal 127 it has the same clock rate as the clock signal129. As previously noted, however, the control signal 127 and clocksignal 129 have different wavelengths. Segment 136 is selected to have acontrollable parameter based on wavelength; thus, fiber loop 131 isformed from dispersive fiber. The wavelength of the control signal isselected to be at a wavelength which transits segment 136 at a ratewhich is different from that of the wavelength of clock signal 129. Thecontrol signal 127 wavelength and the clock signal wavelength are chosenwith the fiber dispersion to give sufficient differential travel speedsuch that the control and clock pulses completely traverse one anotherover the length of fiber 136. For example, we assume that the controlsignal is selected to be at the "fast" wavelength; hence, the clocksignal must enter the loop 131 first. Thus, the control signal cantraverse or "slip" past the "mark" signal within the length of segment136, even though the "mark" signal precedes the entrance of the controlsignal into segment 136.

The optional wavelength filter 142 further prevents any portion of thedeteriorated optical data signal 127, which is not removed by coupler137, from corrupting the regenerated data signal 141. Optical amplifier143 amplifies the regenerated data signal to the proper level fortransmission over fiber link 145. Optical amplifier 143 may be an erbiumamplifier or other type of optical amplifier.

It should be noted again that in our example, the wavelength of theregenerated data signal 204 (e.g., λ1) outputted to fiber link 145 isdifferent from the wavelength of the deteriorated optical input datasignal 203 received over fiber link 111 (e.g., λ2). Thus, regenerator150 can be viewed as a circuit which converts data signals at wavelengthλ1 to data signals at wavelength λ2. If a communication system needs tworepeaters and if the λ1 to λ2 repeater 120 is followed by a λ2 to λ1repeater (not shown), then the wavelength of the data signal will berestored to λ1.

The ability of repeater 120 to replicate input data signal 127 is due tothe ability of the Sagnac switch 130 to absorb both amplitude jitter andtiming jitter in the loop 131. The insensitivity to timing jitter iscontributed by the speed differential between signals travelling atdifferent speeds in loop 131. This has been thoroughly described in thepreviously referenced patent application of Gabriel et al. The toleranceto timing jitter is achieved by controlling the dispersion of the fiberin the loop, and the length of fiber between segments. The timing jittertolerance should be chosen large enough to absorb timing jitter in theclock laser source 125, as well as any timing mismatches due to slighterrors in the length of the delay around the Sagnac switch 130.

The insensitivity to amplitude jitter is due to the sinusoidaldependence of the transmission on the control energy due to theinterference which takes place in the Sagnat switch interferometer. Theenergy of the logic 0 pulse should be controlled to produce a nonlinearphase shift in the Sagnat loop which is substantially less than the π/4radians. The energy of the logic 0 pulses is determined in part byimperfections in the fiber circuit (such as an imperfect splitting ratioof the 50:50 coupler of the Sagnac, or imperfections in the polarizationproperties of the polarization sensitive couplers in the Sagnac switch),so can generally be kept low by careful construction of the circuit. Theenergy of the logic 1 pulses should be sufficient to give a nonlinearphase shift which is between approximately π and 3π/2, to ensure thatthe switch operates in a stable regime.

It should be noted that other well-known clock signal recovery circuitsmay be utilized. Also, the invention may be utilized in communicationsystems which utilize an optical transmission medium other than opticalfiber, for example, a free-space medium. Additionally, the function ofSagnac switch 130 may be implemented using a Mach-Zehnder switch orother interferometer switch, using the Kerr effect, and arranged in ananalogous manner to that disclosed herein. A polarization controller canbe added to the data input port or to the clock input port to preventpolarization wander. In another repeater arrangement, it may bedesirable to include a preamplifier and equalizer at the input of therepeater to increase the amplitude and reshape the input data to improvethe operating capabilities of the repeater. In an alternate embodiment,the clock signal 129 can utilize one polarization signal and the controlsignal CII, I can be an orthogonal polarization signal. The fiber loopsegment 136 may then be a polarization maintaining fiber.

What has been described is merely illustrative of the application of theprinciples of the present invention. Other arrangements and methods canbe implemented by those skilled in the an without departing from thespirit and scope of the present invention.

We claim:
 1. An optical signal regenerator arrangement comprising meansfor recovering an optical clock signal from a received data signal;meansfor injecting said clock signal into a controllable propagation speedmedium, to develop a mark signal that travels in said medium in onedirection and a reference signal that travels in said medium in theopposite direction; means for controllably injecting said received datasignal into said medium, that travels through said medium in thedirection of said mark signal, the timing of said data signal inrelation to said mark signal controlled so that the data signal and themark signal traverse each other while the mark signal is travelingthrough said medium; and means for combining said reference signal afterits travel through said medium and said mark signal after its travelthrough said medium to generate a reconstructed data signal from saidreceived data signal.
 2. The arrangement of claim 1 wherein said meansfor combining is interferometric combining means.
 3. The arrangement ofclaim 2 wherein said combining means develops an output signal at anoutput port when the signals applied to said combining means interfereconstructively.
 4. The arrangement of claim 2 wherein said combiningmeans develops an output signal at a first output port when the signalsapplied to said combining means interfere constructively and an outputsignal at a second output port when the signals applied to saidcombining means interfere destructively.
 5. The arrangement of claim 1wherein said means for controllably injecting includes one or more ofthe following: means for injecting a control signal approximately when amark signal is present in said transmission medium, means for injectinga control signal when a mark signal is absent in said transmissionmedium, and means for abstaining from injecting a signal when a marksignal is present in said transmission medium.
 6. The arrangement ofclaim 1 wherein said controllable propagation speed medium ischaracterized by a propagation speed that is a function of acontrollable signal parameter of said control signal.
 7. The arrangementof claim 6 wherein said controllable signal parameter is the wavelengthof the control signal.
 8. The arrangement of claim 7 wherein said clocksignal is one wavelength and said control signal is of a differentwavelength.
 9. The arrangement of claim 6 wherein said controllablepropagation speed medium has a non-linear index of refraction.
 10. Thearrangement of claim 6 wherein said controllable signal parameter iscontrolled to produce a phase shift on each mark signal as it travelssaid medium with said control signal such that it is altered bysubstantially π radians from the phase shift of said mark signal as ittravels said medium without said control signal.
 11. The arrangement ofclaim 6 wherein said controllable signal parameter is the intensity ofthe control signal.
 12. The arrangement of claim 1 wherein in thecontrollable progapation speed medium in said transmission mediumsignals of different wavelengths propagate at different speeds andwherein the propagation speed for a first wavelength signal when asecond wavelength signal is present differs from the propagation speedfor that first wavelength signal when the second wavelength signal isabsent.
 13. The arrangement of claim 1 wherein said mark signal ispolarized and said control signal is polarized, and the polarization ofsaid mark signal is orthogonal to the polarization of said controlsignal.
 14. The arrangement of claim 1 wherein said means for injectinga signal is a four-port coupler having its first port connected as aninput port, its second and fourth ports connected to, respectively, afirst and second ends of said transmission medium, the ports being bothinput and output ports, and its third port forming an output port ofsaid arrangement.
 15. The arrangement of claim 1 further comprising acoupler connected to said transmission medium for extracting the controlsignal traveling in said medium.
 16. An optical communication systemcomprisingmeans for transmitting an optical data signal over an opticalcommunication medium producing a deteriorated optical data signal andmeans for regenerating the transmitted data signal from saiddeteriorated data signal, said regenerator means including means forremoving a clock signal from said deteriorated data signal; means forinjecting said clock signal into a controllable propagation speedmedium, to develop a mark signal that travels in said medium in onedirection and a reference signal that travels in said medium in theopposite direction; means for controllably injecting said deteriorateddata signal into said medium, that travels through said medium in thedirection of said mark signal, the timing of said deteriorated datasignal in relation to said mark signal controlled so that thedeteriorated data signal and the mark signal traverse each other whilethe mark signal is traveling through said controllable propagation speedmedium; and means for combining said reference signal after its travelthrough said medium and said mark signal after its travel through saidmedium to regenerate a data signal from said deteriorated data signal.17. The system of claim 16 whereinsaid regenerating means is part of arepeater apparatus including an amplifier for amplifying saidregenerated data signal.
 18. The system of claim 16 whereinsaidregenerating means is pan of a receiver apparatus including means forconverting said regenerated data signal into an electronic data outputsignal.